Recently, biomass resins obtainable from plant-based components as raw materials have been attracting much attention, as compared with conventional synthetic resins synthesized from petroleum, and various studies have been made of biomass resins. Such plant-derived resins are carbon neutral materials, since, even if they are burned during disposal, plants absorb the generated carbon dioxide to undergo photosynthesis and become raw materials, and replacement of conventional synthetic resins with these plant-derived resins is expected to be a method that contributes to the prevention of global warming. As these resins derived from living organisms, polyhydroxybutyrate, polylactic acid, and the like are known, and among these, polylactic acid has attracted much attention since polylactic acid is advantageous in that lactic acid, lactides, or the like, which are obtainable from plant resources such as corn, can be used as a raw material, and that polylactic acid has thermal plasticity and can be fabricated by melting. However, the melting point of general polylactic acid is about 170° C., and therefore, improvement in heat resistance has been required for their application to resin molded bodies or a synthetic fibers.
Poly-L-lactic acid (hereinbelow, as appropriate, referred to as “PLLA”), which is composed of only an L-lactic acid unit, which is an optical isomer, and poly-D-lactic acid (hereinbelow, as appropriate, referred to as “PDLA”), which is composed of only a D-lactic acid unit, exist as polylactic acids, and it is known and noted that, when these make a pair and are filled in a crystal lattice to generate a stereo complex crystal, an increase in the melting point is realized.
For example, it is known that when PLLA and PDLA are mixed together in the state of a solution or in a molten state, stereo complex crystals are formed (see Japanese Patent Application Laid-Open (JP-A) No. 63-241024, Macromolecules, Vol. 24, pages 5651-5656 (1991) and Polymer, Vol. 49, pages 5670-5675 (2008)). These stereo complex crystals have a melting point higher than that of α crystal (melting point 170° C.) obtainable from a homopolymer of PLLA or PDLA, and exhibit resistance to hydrolysis; however, when the molecular weight of PLLA or PDLA, which is used as a raw material, is high, it is hard to efficiently obtain stereo complex crystals. In addition, there are problems in that the yield of stereo complex crystals differs according to the conditions for preparation such as the molecular weight of the raw material or the mixing temperature, and that it takes a lot of time to grow, and the like, and thus, in practice, stable production of stereo complex crystals has not yet been realized.
Further, an attempt has been made to generate stereo complex crystals having a melting point higher than those of α crystals, by mixing a homopolymer of PLLA and a homopolymer of PDLA at a ratio of 1:1 (see, for example, Macromolecules, Vol. 24, pages 5651-5656 (1991) and Polymer, Vol. 49, pages 5670-5675 (2008)); however, the melting point of the stereo complex crystal to be obtained is 230° C. at highest, and there is still room for improvement in heat resistance in order to enable application to molded bodies or synthetic fibers.
Moreover, a method of melt-mixing or solution-mixing copolymers containing an L-lactic acid block and a D-lactic acid block, the copolymers having different composition ratios from each other, to prepare a polylactic acid having a high content of stereo complex crystals has been proposed (see, for example, JP-A No. 2007-191625). However, in this production method, the processes are complicated in that, first, plural copolymers containing a L-lactic acid block and a D-lactic acid block are prepared, and the melting point of the obtained polylactic acid is from 147° C. to 211° C., and therefore, the obtained polylactic acid is less likely to be applied to a material for a molded body or the like that needs to have heat resistance.
For this reason, a technique for stably and efficiently producing a polylactic acid that contains stereo complex crystals at a high ratio and has excellent heat resistance is required.